This application claims priority to U.S. Provisional Application No. 60/319,430, filed Jul. 29, 2002, the entirety of which is incorporated herein by reference.
The present invention relates to an automatic amplitude control loop that sets the current flowing through a voltage controlled oscillator (VCO) at an optimal level over process and temperature variations.
VCOs are used in radio communication devices to generate a desired signal frequency for up-mixing signals in the transmission process or down-mixing signals In the reception process. Implementing a VCO in an Integrated circuit (IC) is an important challenge in the design of a transceiver IC. One performance characteristic of a VCO is variation in the amplitude of the oscillator voltage. As a result, VCO design work includes various circuits, called automatic amplitude control circuits, to control the oscillator voltage amplitude. The VCO automatic amplitude control circuit should operate without degrading performance of other elements in the VCO. While amplitude control circuits are known in the art, many of these circuits negatively affect operation of the VCO.
What is needed is an VCO automatic amplitude control circuit that is easy to implement and does not impact performance of the VCO.
Briefly, a VCO is provided having an automatic amplitude control circuit in the form of a sensing amplifier provided in the feedback loop to sense the oscillator amplitude and draw current away only when the positive peak voltage is above a certain value. In general, any amplifier may be used in the feedback loop that outputs current proportional to a peak positive input (in a non-linear and asymmetric fashion with respect to the changing voltage). In one example, the amplifier in the feedback loop comprises first and second transistors that are set nominally in cut off and behave as class C amplifiers. The advantage of this amplifier transistor configuration is that the amplifier they form does not load the LC tank circuit and has a high input impedance.